1. Field of the Invention
The present invention relates to accelerated data acquisition for magnetic resonance imaging, and more particularly, to a compressed sensing reconstruction apparatus and method for imaging of the blood flow and angiography in the human body.
2. Description of the Related Art
Phase contrast (PC) cardiac magnetic resonance (CMR) imaging is widely used in the prior art for the clinical in-vivo assessment of blood flow in the diagnosing of various diseases and the depiction of the vessels in the body; for example, as described in Nayler et al., “Blood flow imaging by cine magnetic resonance”, J Comput Assist Tomogr 1986; 10(5):715-722; Pelc et al., “Phase contrast cine magnetic resonance imaging”, Magn Reson Q 1991; 7(4):229-254; and Rebergen et al., “Magnetic resonance measurement of velocity and flow: technique, validation, and cardiovascular applications”, Am Heart J 1993; 126(6):1439-1456.
Through-plane aortic and pulmonic blood flow are measured and used for the evaluation of cardiac function and output, mitral regurgitation, and shunts. Clinically, a through-plane 2D image acquisition is performed for the evaluation of blood flow. Recent advances have also enabled 3D time-resolved PC image processing that allows quantification and visualization of the blood flow in all three directions, as described in Markl et al., “Time-resolved three-dimensional phase-contrast MRI”, J Magn Reson Imaging 2003; 17(4):499-506. For quantitative measurement of cardiac indices such as cardiac output or mitral regurgitation, flow measurements should be accurate and reproducible. However, despite the potential of PC CMR as an alternative to Doppler ultrasound for evaluation of these indices, its accuracy is impacted by several limitations including background offset, eddy currents, and long scan time.
One of the challenges of PC CMR is the long scan time which limits spatial and temporal resolution. Several acquisition methods have been used to improve the data acquisition efficiency and reduce the total scan time in PC CMR; that is, increase the acceleration rate of image acquisition and reconstruction. Echo planar imaging has been used to improve the temporal resolution of flow imaging, such as in Ordidge et al., “Real time movie images by NMR”, British Journal of Radiology 1982; 55(658): 729-733. Non-Cartesian k-space trajectories including spiral and radial sequences have also been used to reduce scan time, as described in Nayak et al., “Real-time color flow MRI”, Magn Reson Med 2000; 43(2):251-258; Park et al., “Rapid measurement of time-averaged blood flow using ungated spiral phase-contrast”, Magn Reson Med 2003; 49(2):322-328; and Barger et al., “Phase-contrast with interleaved undersampled projections”, Magn Reson Med 2000; 43(4):503-509.
Parallel imaging methods are also used in the prior art, for example, in “Pruessmann et al., “SENSE: sensitivity encoding for fast MRI”, Magn Reson Med 1999; 42(5):952-962; and Griswold et al., “Generalized autocalibrating partially parallel acquisitions (GRAPPA)”, Magn Reson Med 2002; 47(6):1202-1210. Such parallel imaging methods, which have been widely used clinically for accelerated imaging, were shown to provide accurate flow measurements with reduced scan time, as described in Thunberg et al., “Accuracy and reproducibility in phase contrast imaging using SENSE”, Magn Reson Med 2003; 50(5):1061-1068. The study by Baltes et al., “Accelerating cine phase-contrast flow measurements using k-t BLAST and k-t SENSE. Magn Reson Med 2005; 54(6):1430-1438; showed that k-t BLAST and k-t SENSE are promising approaches for high-resolution breath-hold flow quantification through the ascending aorta with up to 5-fold acceleration.
To overcome the limitation of the acceleration rate of the previous schemes, recent studies have shown that higher acceleration rate can be achieved using compressed-sensing (CS) when compared with more established techniques such as parallel imaging, as described in Tao et al., “Compressed sensing reconstruction with retrospectively gated sampling patterns for velocity measurement of carotid blood flow”, Proceedings of the 18th Annual Meeting of ISMRM. Volume 18. Stockholm, Sweden; 2010. p 4866; Hsiao et al., “Quantitative assessment of blood flow with 4D phase-contrast MRI and autocalibrating parallel imaging compressed sensing”, Proceedings of the 19th Annual Meeting of ISMRM. Volume 19. Montreal, Canada; 2011. p 1190; and Kim et al., “Accelerated phase-contrast cine MRI using k-t SPARSE-SENSE”, Magn Reson Med 2012; 67(4):1054-1064.
Compressed sensing reconstruction with accelerated PC acquisitions is a promising technique to increase the scan efficiency. In a study by Tao et al., “Compressed sensing reconstruction with retrospectively gated sampling patterns for velocity measurement of carotid blood flow”, Proceedings of the 18th Annual Meeting of ISMRM. Volume 18. Stockholm, Sweden; 2010. p 4866”; CS reconstruction is simulated with retrospectively gated 2D PC cine scans of carotid blood flow. The study by Hsiao et al., “Quantitative assessment of blood flow with 4D phase-contrast MRI and autocalibrating parallel imaging compressed sensing”, Proceedings of the 19th Annual Meeting of ISMRM. Volume 19. Montreal, Canada; 2011. p 1190”; assessed the accuracy of flow quantification for 4D phase contrast MRI by comparing parallel imaging and CS.
Kim et al., “Accelerated phase-contrast cine MRI using k-t SPARSE-SENSE”, Magn Reson Med 2012; 67(4):1054-1064; performed a combination of spatio-temporal (k-t)-based CS and parallel imaging, called k-t SPARSE-SENSE for prospectively under-sampled in-vivo data, and reported agreements between k-t SPARSE-SENSE and GRAPPA. While k-t approaches could surpass the acceleration that can be achieved using acceleration only in the spatial domain, k-t approaches could also cause temporal blurring. Stadlbauer et al., “Accelerated time-resolved three-dimensional MR velocity mapping of blood flow patterns in the aorta using SENSE and k-t BLAST”, Eur J Radiol 2010; 75(1): e15-21; demonstrated that 6-fold k-t BLAST shows a reduction in peak velocity compared to rate 2 SENSE, which is caused by temporal blurring.
In a previous study in the prior art involving CS for PC utilizing the complex difference (CD) of two flow-encoded images, described in King et al., “Compressed sensing with vascular phase contrast acquisition”, Proceedings of the 17th Annual Meeting of ISMRM. Volume 17. Honolulu, USA; 2009. p 2817; such an application of CS involved MR angiography with maximum intensity projection images in the brain, and was not applied to or involving flow assessment of blood or substances in living tissue, such as blood through a heart. However, the method of King does not use sparsifying in the complex difference domain, and the method only related to use of CS in addition to parallel imaging for angiography.
In addition, the aim of the method of King is to obtain MR angiography with no utilization of individual flow encoded images m1, m2, m3, and m4. The method of King uses CD images by combining all of such CD images to obtain one flow weighted image, which does not allow for extraction of flow measurements. As described in King, “Complex difference processing was used to calculate the flow images for each axis, e.g. for the x-axis,mx=√{square root over (m1|2+|m2|2−2|m1∥m2|cos [∠(m1m*2)])}etc. where Mecho refers to the image for each flow moment combination (echo=1, 2, 3, 4). The final flow image is given by:mflow=√{square root over (mx+my+mz)}.
Furthermore, the method of King solves equations to process image data, but King states that “we added a sparsifying transform of the final flow image, giving the objective function which is:J=Σecho[∥Emecho−Secho∥22+λ∥Ψmecho∥1]+λflow∥Ψmflow∥1”where “m flow” is an image obtained by squareroot of sum of squares, which does not permit generating flow images or Calculate squareroot of sum of squares and create a MR angiogram.
Therefore, due to the deficiencies in the prior art in image acquisition and reconstruction methods for accurate and reproducible flow measurements at very high acceleration rates, alternative methods are needed that can enable acceleration of image acquisition and processing of blood flow in the human body without temporal blurring.